1. Field of the Invention
The present invention relates to a transflective liquid crystal device (hereinafter also referred to as a transflective liquid crystal device) capable of both reflective display, which reflects incident light to display an image, and transmissive display, which transmits incident light to display an image.
2. Description of the Related Art
The transflective liquid crystal device is widely used as a display device of portable information equipment. FIG. 11 schematically illustrates the structure of a conventional transflective liquid crystal device 1000. The transflective liquid crystal device 1000 includes an absorptive polarizer 1020, a liquid crystal cell 1030, a light diffusing plate 1040, a reflective polarizer 1050, and a light absorbing plate 1060. A backlight 1070 is further disposed outside the light absorbing plate 1060. The liquid crystal cell 1030 includes a lower glass substrate 1033, an upper glass substrate 1031, and a liquid crystal layer 1035 sealed between these glass substrates 1031 and 1033. A plurality of transparent signal electrodes 1034 are mounted on the upper surface of the lower glass substrate 1033. A plurality of transparent scanning electrodes 1032 are mounted to be perpendicular to the plurality of signal electrodes 1034 on the lower surface of the upper glass substrate 1031. The liquid crystal cell 1030 has a passive matrix configuration, in which one pixel is defined by one signal electrode 1034, one scanning electrode 1032, and the liquid crystal layer 1035 between these electrodes 1034 and 1032. Namely the light transmitted through the liquid crystal layer 1035 is modulated according to the voltage applied between one signal electrode 1034 and one scanning electrode 1032. The liquid crystal layer 1035 may be made of a TN (twisted nematic) liquid crystal composition or STN (super twisted nematic) liquid crystal composition. A translucent film having the transmittance of about 50% is used for the light absorbing plate 1060.
FIG. 12 shows problems arising in the conventional transflective liquid crystal device 1000. The absorptive polarizer 1020 has an axis of transmission 1020T that is set parallel to the plane of the drawing, and an axis of absorption 1020A that is perpendicular to the plane of the drawing. The reflective polarizer 1050 has, on the other hand, an axis of transmission 1050T that is parallel to the plane of the drawing, and an axis of reflection 1050R that is perpendicular to the plane of the drawing. The following describes the operations of the liquid crystal display 1000 on the assumption that the polarizing direction of the light transmitted through the liquid crystal cell 1030 is rotated by 90 degrees while no voltage is applied between the signal electrodes 1034 and the scanning electrodes 1032 (that is, when the liquid crystal cell 1030 is in an OFF state).
This liquid crystal device 1000 has two display modes, that is, a reflective display mode using incident light 1100 from the outside and a transmissive display mode using light 1120 emitted from the backlight 1070. In the reflective display mode, when the non-polarized light 1100 enters the absorptive polarizer 1020, a linearly polarized light component having the polarization direction parallel to the axis of absorption 1020A is mostly absorbed by the absorptive polarizer 1020, while only a linearly polarized light component having the polarization direction parallel to the axis of transmission 1020T is transmitted through the absorptive polarizer 1020 and enters the liquid crystal cell 1030. The optical rotatory power of the liquid crystal cell 1030 causes the light component entering the liquid crystal cell 1030 to be converted into linearly polarized light having a polarizing direction that is perpendicular to that of the incident light. The polarizing direction of the light emitted from the liquid crystal cell 1030 is substantially identical with the direction of the axis of reflection 105OR of the reflective polarizer 1050, so that most of the light emitted from the liquid crystal cell 1030 is reflected by the reflective polarizer 1050 and re-enters the liquid crystal cell 1030 as return light. The liquid crystal cell 1030 converts the return light into linearly polarized light having a polarizing direction that is perpendicular to that of the return light. At this moment, the polarizing direction of the return light emitted from the liquid crystal cell 1030 is substantially identical with the direction of the axis of transmission 1020T of the absorptive polarizer 1020, so that most of the return light emitted from the liquid crystal cell 1030 is transmitted through the absorptive polarizer 1020. In the reflective display mode, the pixels where the liquid crystal cell 1030 is in the OFF state receive the light reflected and returned as discussed above and are thereby observed as bright pixels. The pixels where the liquid crystal cell 1030 is in an ON state are, on the contrary, observed as dark pixels.
In the transmissive display mode, on the other hand, when the non-polarized light 1120 enters the reflective polarizer 1050, a linearly polarized light component having the polarization direction parallel to the axis of reflection 105OR is mostly reflected by the reflective polarizer 1050, while only a linearly polarized light component having the polarization direction parallel to the axis of transmission 1050T is transmitted through the reflective polarizer 1050 and enters the liquid crystal cell 1030. The optical rotatory power of the liquid crystal cell 1030 causes polarizing direction of the light transmitted through the liquid crystal cell 1030 to be converted into a direction substantially parallel to the axis of absorption 1020A of the absorptive polarizer 1020. Most of the light emitted from the liquid crystal cell 1030 is accordingly absorbed by the absorptive polarizer 1020 and is not transmitted through the absorptive polarizer 1020. In the transmissive display mode, since the light is absorbed in the course of the optical path, the pixels where the liquid crystal cell 1030 is in the OFF state are observed as dark pixels. The pixels where the liquid crystal cell 1030 is in the ON state are, on the contrary observed as bright pixels. The relationship between the ON/OFF state of the liquid crystal cell 1030 and the bright/dark state of the pixel in the transmissive display mode is reverse to that in the reflective display mode. In the transflective liquid crystal device 1000, the brightness and darkness of display are reversed between the reflective display mode and the transmissive display mode.